Well water in Yellowstone County, Montana
1in15
Testing recommended
tested well samples in Yellowstone County exceeded the EPA limit for nitrate (10 mg/L), based on public data from USGS Water Quality Portal.
The strongest signal is uranium: 7% of 15 local samples were above the limit, too few samples to carry the headline but first on the test list below.
1,135 test results across 11 contaminants from at least 201 wells, 1914 to 2012; chloride has the most at 209. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 12,405 people in Yellowstone County get their drinking water from a private well, per USGS 2015 county water use. That is about 8% of residents.
Separately, the USGS national arsenic model estimates that 5% of the 12,405 people on private wells in Yellowstone County draw water above 10 ug/L (the model's range is 4% to 6%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Yellowstone County. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.
| Contaminant | Above limit | Samples | Detected | Median | Highest | Limit |
|---|---|---|---|---|---|---|
| Uranium 2011 to 2012 | 15 | 100% | 10.7 | 60.8 | 30 ug/L EPA limit | |
| Nitrate 1914 to 2012 | 196 | 96% | 0.82 | 56 | 10 mg/L EPA limit | |
| Arsenic 1978 to 2012 | 17 | 88% | 2.6 | 20.1 | 10 ug/L EPA limit | |
| Fluoride 1972 to 2012 | 154 | 99% | 0.55 | 5.8 | 4 mg/L EPA limit | |
| Lead 1978 to 2012 | 18 | 17% | 0 | 0.618 | 15 ug/L EPA limit | |
| Copper 1978 to 2012 | 17 | 76% | 0.002 | 0.057 | 1.3 mg/L EPA limit | |
| Chloride 1914 to 2012 | no limit | 209 | 100% | 20 | 1,200 | 250 mg/L nuisance level |
| Iron 1916 to 2012 | no limit | 198 | 89% | 95 | 19,000 | 300 ug/L nuisance level |
| Sodium 1972 to 2012 | no limit | 155 | 100% | 280 | 2,100 | 20 mg/L health guideline |
| Manganese 1972 to 2012 | no limit | 154 | 66% | 10 | 1,920 | 300 ug/L health guideline |
Sampled fewer than 10 times, too few to summarize: E. coli (2).
Over time
Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.
| Contaminant | before 2000 | 2000s | 2010s | 2020s |
|---|---|---|---|---|
| Nitrate | 5%166 samples | n/a | 13%30 samples | n/a |
| Fluoride | 4%137 samples | n/a | 0%16 samples | n/a |
Nitrate above the limit in Yellowstone County rose from 5% of 166 samples before 2000 to 13% of 30 in the 2010s.
Closer than the county
See the sampled wells near your address
The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.
Check my addressWhat this means
The contaminants most often above their limit here are uranium (7% of 15 samples), nitrate (7% of 196 samples), and arsenic (6% of 17 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.
Chloride, Iron, and Manganese are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
What to test for in Yellowstone County
- 1Uranium7% of 15 local samples were above the limit.
- 2Nitrate7% of 196 local samples were above the limit.
- 3Arsenic6% of 17 local samples were above the limit.
- 4Fluoride4% of 154 local samples were above the limit.
- 5Coliform bacteriaRecommended for every private well, every year.
- 6ChlorideDetected in 100% of local samples.
- 7IronDetected in 89% of local samples.
- 8Volatile organic compounds (VOCs)464 leaking underground storage tank sites on record locally.
Test your Yellowstone County well for uranium, nitrate and arsenic
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.
Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.
Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.
Where to get a well tested in Montana: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Yellowstone County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Yellowstone County
Not a principal aquifer
The USGS map places this area outside any principal aquifer, which usually means wells here draw from fractured bedrock or thin local deposits rather than a large regional aquifer. Bedrock wells of that kind are where arsenic, radon and uranium turn up, and yields vary from well to well.
Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the county centroid. A single well can sit in a different unit, especially near a boundary.
Official help in Montana
Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.
- Water Resources, Montana DEQ
- Water Laboratory Certification Program, Montana DPHHS
Contamination sources on record in Yellowstone County
Montana DEQ records 464 leaking underground storage tank sites in Yellowstone County, among 640 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.
- 464
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 159
- registered underground storage tank sitesRegistered tanks are regulated and inspected; they matter most when they are old or near a shallow well.
- 4
- lined landfills and transfer facilities
- 13
- other solid waste facilities
Treatment that fits the local profile
Uranium
Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Fluoride
Point-of-use reverse osmosis; Activated alumina; Distillation
Iron
Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration
Systems we would look at first
Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.
For uranium
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For uranium
Whole-house arsenic and uranium filter
whole-house, at the point of entry, about $2,500
Adsorptive iron-based media; sized to your well yield.
For nitrate
Aquasana SmartFlow reverse osmosis
under-sink or countertop, about $225
Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.
For iron
Iron and manganese oxidizing filter
whole-house, at the point of entry, about $1,800
Air-injection or greensand media; fixes staining at the source.
Free, no obligation
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Common questions
Is there arsenic in well water in Yellowstone County?
Arsenic was detected in 88% of 17 public well samples in Yellowstone County, and 6% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.
How often should I test a private well in Yellowstone County?
Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.
Counties near Yellowstone County
Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.
Want the numbers themselves? Download the Montana dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 1,135 samples in Yellowstone County. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
- USGS national arsenic model (Ayotte 2017): estimated share of 12,405 people on private wells in Yellowstone County above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.
- Montana DEQ regulated sites: 640 regulated sites located in Yellowstone County. Petroleum tank releases, underground storage tank facilities and licensed solid waste facilities from the Montana Department of Environmental Quality. Retrieved September 18, 2026.
Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.